The IMS (IP Multimedia Subsystem) is a sub-system of the UMTS (Universal Mobile Telecommunication System) system, introduced in UMTS Release 5, intended to allow dynamic establishment and control of multimedia sessions between at least two items of equipment (for example between two user terminals or between a user terminal and an application server).
The establishment of a multimedia (for example audio, video and/or textual) session between two items of equipment, for example two mobile terminals, respectively calling and called, in the IMS is performed in two stages:
1. Primary Signaling PDP Context
The calling equipment activates a first signaling session, called the primary signaling PDP context. During this signaling session, the calling and called items of equipment exchange signaling data so as to negotiate the characteristics of the call (in particular the types of coding supported).
2. Secondary Data Transport PDP Context
After the phase of negotiating the characteristics of the call, the calling item of equipment activates a second session, called the secondary media data transport PDP context, intended for the transport of data relating to the call proper. It may for example involve a video and audio data transport session, consultation of a multimedia server or a simple voice over IP session.
The term “media data” is understood to refer to the data specific to a communication, in particular audio, video and/or textual data, different from the signaling data already exchanged in the primary PDP context.
During the activation of a PDP context (signaling data or media data transport) on the initiative of an item of equipment, the radio access network of the UMTS (UTRAN), and more particularly the home RNC (Radio Network Controller) to which the equipment is tied, allocates network resources to it by activating a communication channel, commonly called an “RAB” (Radio Access Bearer Parameters), at the radio access network level. The RAB is defined by a set of quality of service parameters corresponding overall to those of the quality of service profile of the activated PDP context.
Subsequently, the term “data transport channel” will be used to refer both to a PDP context and to an RAB.
The activation of any channel, whether it involves a PDP context or an RAB, for signaling data transport or for media data transport, makes it necessary to negotiate beforehand the quality of service parameters related to this channel. The present invention is more particularly concerned with the activation of a signaling channel (that is to say of a signaling data transport channel), negotiation of the quality of service parameters and prioritization at the transport level of such a channel.
It is paramount that the activation of the primary signaling PDP context, in the course of which the signaling parameters for the secondary call-related media data transport PDP context are negotiated, be performed correctly, failing which this secondary context cannot be activated. It ought therefore to be processed in a priority manner. For this purpose, the 3GPP (3rd Generation Partnership Project) standard has introduced a parameter SI (Signaling Indication) indicating whether the packets which will be transmitted in the PDP context do or do not belong to signaling, depending on whether it is associated with the value “Yes” or “No”. This parameter SI forms part of the set of parameters specifying the quality of service that are requested by an item of equipment wishing to open a PDP context. When an item of equipment wishes to open a PDP signaling context, it indicates: “SI=Yes”.
The 3GPP standard advocates using the quality of service parameter THP (Traffic Handling Priority) when the parameter SI=YES. This parameter THP, related to the “Interactive” traffic class, makes it possible to differentiate the interactive sessions with respect to one another from the quality of service standpoint. Each interactive session is associated with a parameter THP that can take the value 1, 2 or 3, the value 1 corresponding to the highest priority level. Stated otherwise, the standard suggests, when the parameter SI is activated (SI=Yes), that the value of the parameter THP be fixed at 1. The PDP signaling context thus exploits the priority level due to the value 1 of the parameter THP. However, if during the activation of a new primary signaling PDP context, PDP contexts corresponding to interactive sessions, having a parameter THP equal to 1, are already activated or undergoing activation, the network processes the activated PDP contexts corresponding to the interactive sessions in progress and the PDP signaling context in the same manner.